The invention relates to an optical waveguide fiber and a method of making a waveguide fiber, having a refractive index profile which varies in both the radical and azimuthal directions. The additional flexibility afforded by the azimuthal variation provides for index profile designs which meet a larger number of waveguide fiber performance requirements than is possible using refractive index variation in only the radical coordinate direction.
Recent development of waveguide fibers having refractive index profiles which vary in the radial direction has shown that particular properties of the waveguide can be optimized by adjusting this profile. Varying the refractive index profile in a more general way than, for example, a simple step, allows one to select the value of one or more waveguide properties without sacrificing a base set of properties including attenuation, strength, or band resistance.
In addition, certain azimuthal asymmetric core refractive index profiles, such as those having elliptical, triangular, or square core geometry have been shown to provide useful waveguide properties such as preservation or mixing of the polarization modes.
It is expected, therefore, the core refractive index profiles which vary in both the azimuthal and radial direction will offer the opportunity to fabricate waveguides having new or improved properties for use in telecommunication, signal processing, or sensor systems.
In U.S. Pat. No. 3,909,110, Marcuse, (""110 patent) an azimuthally asymmetric core of a multimode waveguide is described. A calculation in the ""110 patent indicates that periodic variations in index in both the radial and azimuthal directions would cause mode coupling, thereby increasing bandwidth, while limiting losses due to coupling to radiation modes. The concept was not extended to include single mode waveguides. Also the scope of the ""110 patent is quite limited in that it refers only to sinusoidal azimuthal variations.
In describing the present azimuthally and radially asymmetric core, the concept of core sectors is introduced. A core sector is simply a portion of the core which is bounded by a locus of points of a first and a second radius which form an annular region in the waveguide. Each of the radii are different one from another and are less than or equal to the core radius. The remaining boundaries of a sector are two planes oriented at an angle with respect to each other and each containing the waveguide fiber centerline. A change in refractive index along a line within a sector means the refractive index is different between at least two points along the line.
The following definitions are in accord with common usage in the art. A segmented core is a core which has a particular refractive index profile over a pre-selected radius segment. A particular segment has a first and a last refractive index point. The radius from the waveguide centerline to the location of this first refractive index point is the inner radius of the core region or segment. Likewise, the radius from the waveguide centerline to the location of the last refractive index point is the outer radius of the core segment.
The relative index, xcex94, is defined by the equation, xcex94=(n12-n22)/2n12, where n1 is the maximum refractive index of the index profile segment 1, and n2 is a reference refractive index which is taken to be, in this application, the minimum refractive index of the clad layer. The term xcex94%, which is 100X xcex94, is used in the art.
The term refractive index profile or simply index profile is the relation between xcex94% or refractive index and radius over a selected portion of the core. The term xcex1-profile refers to a refractive index profile which follows the equation.
n(r)=n0(1xe2x88x92xcex94[r/a]xcex1) where r is core radius, xcex94is defined above, a is the last point in the profile, r is chosen to be zero at the first point of the profile, and xcex1 is an exponent which defines the profile shape. Other index profiles include a step index, a trapezoidal index and a rounded step index, in which the rounding is typically due to dopant diffusion in regions of rapid refractive index change.
In a first aspect of the invention, a single mode waveguide has a core having at least one sector. The refractive index of at least one point within the sector is different from that of at least one point outside the sector. In the case where the sector is exactly half the core, the choice of what constitutes a point inside the sector can be chosen arbitrarily without any loss of precision of definition of the profile. The core refractive index profile changes along at least a portion of one radius to provide radial asymmetry. At a pre-selected radius the core refractive index within the sector is different from that outside the sector to provide azimuthal asymmetry.
In one embodiment, the overall core has cylindrical symmetry and thus is conveniently described in cylindrical coordinates, radius r, azimuth angle xcfx86, and centerline height z. The pre-selected radius portion, xcex94r along which the refractive index changes is in the change 0 less than xcex94rxe2x89xa6r0, where r0 is the core radius. The pre-selected radius at which the refractive index is different for at least two different choices of azimuth angle is within this same range.
In another embodiment the pre-selected radius portion is a segment defined as xcex94r=r1-r1, where, 0xe2x89xa6r1 less than r2 and r2 less than r0.
In yet another embodiment, the refractive index changes along any or all radii within a sector, in which the sector has included angle xcfx86 greater than zero but less than or equal to 180xc2x0.
In another embodiment the radius portion is in the range 0 less than xcex94rxe2x89xa6r0, and the azimuth angle xcfx86 and height z have any value provided the coordinate point (r, xcfx86, z) is in the core region.
Further embodiments of the invention include those in which the number of sectors and the angular and radial size of the sectors are specified and the functional relationship between radius r and relative index percent xcex94% is specified. Examples of the functional relationships are the xcex1-profile, the step and rounded step index profiles, and the trapezoidal profile.
Yet further embodiments of the invention include waveguides having a segmented core and a specified number of sectors which include areas in which glass volumes of a particular size and shape have been embedded. Three and four sector embodiments having a particular core configuration and embedded portions are described below. In some embodiments, the embedded portions themselves have a segmented refractive index configuration.
In general the embodiments of this first aspect of the invention can be either single mode or multimode waveguide fibers.
A second aspect of the invention is a method of making an azimuthally and radially asymmetric waveguide fiber. The method may be employed to make either single mode or multimode waveguide fiber.
One embodiment of the method includes the steps of modifying the shape of a draw perform and then drawing the preform into a waveguide fiber having a circular cross section. The shape of the preform is thus transferred to the cylindrically symmetric features contained within the preform, specifically the cylindrical symmetric core features. The draw preform shape may be changed by any of several methods such as etching, a sawing, drilling, or grinding.
In an embodiment of the method, the preform is altered by forming holes or surface indentations therein. Subsequent drawing of the altered preform into a waveguide fiber of circular cross section causes a circularly symmetric core to become radially or azimuthally asymmetry.
In yet another embodiment of the method, two or more core preforms are fabricated and inserted into a glass tube to form a preform assembly. The waveguide fiber resulting from drawing the preform assembly has the asymmetry of the assembly. Spacer glass particles or rods may be incorporated into the tube-core preform assembly.